It ain’t easy being a dolphin.

That’s my takeaway after looking through a database of 1,000 photographs of Common Bottlenose Dolphin dorsal fins released by researchers as part of a 2018 research project. The photographs tell of a violent and brutal life that differs from the idyllic existence we probably imagine when thinking of these delightful mammals.

Scars, scratches and wounds abound in these photos as encounters with unknown creatures and boat propellers leave their marks, imprinting a story of close escapes and cheating death. Some fins are pierced with radio trackers, and others have numbers inscribed into the dolphin's skin by researchers for identification purposes.  

But each dolphin fin’s combination of gouges, nicks and marks does have an unexpected upside for these creatures. They are unique enough to form a sort of fingerprint that can be used to identify distinct dolphins, allowing researchers to track their movements over time.

The Southeast Fisheries Science Center (SEFSC), based in Miami, Florida is part of the National Oceanic and Atmospheric Administration (NOAA) with a mission to provide “...the scientific advice and data needed to effectively manage the living marine resources of the Southeast region and Atlantic high seas”. The research project that involved this data collection was conducted by the SEFSC’s Beaufort, NC facility, which is the second-oldest federal marine laboratory (Woods Hole in MA is the oldest).

The goal of the research project was to provide an updated count of the number of common bottlenose dolphins along the southern coast and estuarine area of North Carolina.

The US Marine Mammal Protection Act (MMPA) is a law that protects all marine mammals which defines large observed migratory populations into 61 distinct “stocks” and corresponding management zones in U.S. waters. The common bottlenose dolphins that were observed from this study occupy two overlapping stocks – the Northern North Carolina Estuarine System Stock and the Southern North Carolina Estuarine System Stock.

The data was collected in January of 2018 on three separate missions, along the coast near Cape Fear, NC. The good news is that this study found the estimated population to be three times higher than the previous estimate from 2006.

After the data collection, the photographs were analyzed to identify individual dolphins. Turns out there is an app for that: FinFindR, which is a code library “for doing photo recognition on dolphins, sharks etc.”

The fin images contain a lot of information. The fins’ shape, color variations, notches, scars and “tooth rakes” all help to distinguish an individual. The Beaufort lab had been compiling photos of these fins since 1995, in a catalog of 2,423 individual dolphins.

Researchers were able to check the 2018 photos against this catalog which provided 228 matches for previously identified dolphins. After completing the analysis, the study had identified 547 distinct dolphins from their photographs. This collection of 1,011 photos includes the photographs from the 2018 research mission as well as the earlier photos of matched dolphins from the Beaufort archives.

The study also looked for the presence of “Xenobalanus globicipitis”, a parasitic barnacle found on many fins.

Tursiops truncatus

Common bottlenose dolphins (Tursiops truncatus) can live to be 40-60 years old, and can be found all around the world, in both tropical and temperate waters. In the U.S., this well-studied intelligent species can be found all throughout the Atlantic coast, the Gulf of Mexico and the Caribbean as well as along the Pacific coast from Washington to California. Measuring from six to thirteen feet and weighing up to 1,400 lbs, these dolphins can be seen in open ocean, near the coastline or in estuaries, bays and harbors.

The bottlenose’s close proximity to human beings creates one of its greatest threats. Becoming entrapped in fishing gear is one of the major threats affecting the populations of bottlenose around the world, as well as contamination from industrial pollution and harmful algae blooms.  

In five of the population stocks along the Atlantic coast, bottlenose populations are classified as “depleted”.

A Dolphin’s Social Network

In addition to simply counting and identifying the dolphins, the researchers set out to understand the social networks that the dolphins adhered to.

The final research paper was published in August of 2022. While studying these two large “stock” areas, the analysis found that there appeared to actually be three distinct populations. The paper concludes that simply counting dolphins in a time and place may not give us enough information to properly define a group of these highly social animals, and that analyzing their own networks may be the best way to manage and protect these fascinating creatures.

Research paper: Hohn AA, Gorgone AM, Byrd BL, Shertzer KW, Eguchi T (2022) "Patterns of association and distribution of estuarine-resident common bottlenose dolphins (Tursiops truncatus) in North Carolina, USA." PLoS ONE 17(8): e0270057. https://doi.org/10.1371/journal.pone.0270057